Hydrogen evolution reaction catalysts, electrodes and electrolyzers based thereon and methods of fabrication thereof

ABSTRACT

The invention provides, in some aspects, methods for fabricating an electrode comprising a nickel/molybdenum (NiMo) hydrogen evolution reaction catalyst on a carbon support, e.g., for use as a cathode in an electrolyzer. A catalyst of the type described above can be prepared by co-precipitation of nickel and molybdenum oxide species on the carbon support followed by its reduction through heat treatment in the presence of nitrogen. Such a catalyst can alternatively be prepared through the thermal degradation of metal-organic complexes of nickel and molybdenum in the presence of the carbon support. Further aspects of the invention comprise a cathode, e.g., for an anion exchange membrane electrolyzer, comprising a nickel/molybdenum hydrogen evolution reaction catalyst as described above. Still further aspects of the invention comprise an anion exchange membrane electrolyzer with a cathode as described above.

BACKGROUND OF THE INVENTION

This application claims the benefit of U.S. Provisional Patent Application Serial No. 63/337,076, filed Apr. 30, 2022, titled “Hydrogen Evolution Reaction Catalysts, Electrodes and Electrolyzers Based Thereon and Methods of Fabrication Thereof,” the teachings of which are incorporated herein by reference.

Demand for hydrogen is expected to rapidly increase with the widespread adoption of hydrogen-air fuel cells in stationary and mobile applications. The inherent contradiction of this emerging paradigm is that while generating electricity with a fuel cell yields no greenhouse gases, an overwhelming percentage of hydrogen is produced using greenhouse gas-producing fossil fuels or their derivatives as feedstock. See, Lehner, F. and D. Hart, Chapter 1—The importance of water electrolysis for our future energy system, in Electrochemical Power Sources: Fundamentals, Systems, and Applications, T. Smolinka and J. Garche, Editors. 2022, Elsevier. p. 1-36.

Hydrogen production via electrolysis is a process where a current is applied to an aqueous electrolyte solution and the water is split into its oxygen and hydrogen components. Electrolysis is a mature technology that has its origins in the late 1800s and used liquid alkaline electrolytes. The introduction of the proton-exchange membrane (PEM) Nafion in the 1960′s allowed for membrane-based electrolyzers that were more compact, scalable and the hydrogen evolved was easier to pressurize. The main drawback to PEM-based electrolyzers is that the cost of the noble-metal anodic oxygen evolution reaction (OER) and cathodic hydrogen evolution reaction (HER) catalysts are too high for widespread adoption. The relatively recent introduction of anion exchange membranes (AEM) allows for cheaper and more abundant metals to be used in electrochemical reactors that would otherwise corrode at higher pHs.

Of the non-noble metal HER catalysts, nickel is a common primary component, support medium, or both. A widely applied strategy to increase HER actively is by alloying it to another metal such as selenium, iron, and molybdenum. See, Xu, P., et al., Co doped Ni0.85Se nanoparticles on RGO as efficient electrocatalysts for hydrogen evolution reaction. Applied Surface Science, 2019. 494: p. 749-755); Wang, P.-c., et al., NiFe Hydroxide Supported on Hierarchically Porous Nickel Mesh as a High-Performance Bifunctional Electrocatalyst for Water Splitting at Large Current Density. ChemSusChem, 2019. 12(17): p. 4038-4045); McKone, J. R., et al., Ni-Mo nanopowders for efficient electrochemical hydrogen evolution. ACS catalysis, 2013. 3(2): p. 166-169. Another broadly used strategy for improving electrochemical performance is to load the catalyst on a support medium such as carbon. Carbon supports are useful because of their low cost, durability, electrical conductivity, and high surface area. See, Lam, E. and J. H. Luong, Carbon materials as catalyst supports and catalysts in the transformation of biomass to fuels and chemicals. ACS catalysis, 2014. 4(10): p. 3393-3410.

The use of metal-organic chelates in the preparation of HER catalyst has been explored in a paper by Doan et al. See, Doan, H., et al., Functionalized Embedded Monometallic Nickel Catalysts for Enhanced Hydrogen Evolution: Performance and Stability. Journal of The Electrochemical Society, 2021. 168(8): p. 084501. Disadvantages of Doan's technique is that it does not lend itself to application with multiple metallic chelates nor is it amenable to industrial scale up.

Objects of the invention are to provide improved catalysts to promote hydrogen evolution reactions.

Related objects are to provide improved electrodes (e.g., cathodes) and electrolyzers based on such catalysts.

Further objects of the invention are to provide methods of fabrication of such catalysts, electrodes and electrolyzers.

SUMMARY OF THE INVENTION

The foregoing are among the objects attained by the invention, which provides in some aspects, methods for fabricating an electrode comprising a nickel/molybdenum (NiMo) hydrogen evolution reaction catalyst on a carbon support, e.g., for use as a cathode in an electrolyzer.

According to related aspects of the invention, a catalyst of the type described above is prepared by co-precipitation of nickel and molybdenum oxide species on the carbon support followed by its reduction through heat treatment in the presence of hydrogen.

According to other related aspects of the invention, the catalyst is fabricated through the thermal degradation of metal-organic complexes of nickel and molybdenum in the presence of the carbon support.

Further aspects of the invention comprise a cathode, e.g., for an anion exchange membrane electrolyzer, comprising a nickel/molybdenum hydrogen evolution reaction catalyst as described above.

Other aspects of the invention provide a method of preparing a hydrogen evolution reaction catalyst comprising steps of (A) co-precipitating nickel and molybdenum oxides on a carbon support to form a catalyst precursor, and (B) annealing the catalyst precursor in the presence of hydrogen to reduce the nickel and molybdenum oxides and, thereby, to yield a hydrogen evolution reaction catalyst.

Related aspects of the invention provide a method, e.g., as described above, wherein step (A) comprises forming aminated salts of nickel and molybdenum.

Further related aspects of the invention provide a method, e.g., as described above, wherein step (A) comprises preparing a solution by adding the aminated salts to a dispersion of the carbon support in a solvent.

Yet further related aspects of the invention provide a method, e.g., as described above, comprising heating the solution to precipitate the nickel and molybdenum oxides onto the carbon support.

Still yet further aspects of the invention provide a method, e.g., as described above, comprising forming the catalyst into an electrode.

Related aspects of the invention provide an electrode comprising a catalyst prepared according to the methods above.

Still other aspects of the invention provide an electrolyzer comprising a cathode formed of a catalyst prepared as discussed above.

Other aspects of the invention provide a method of preparing a hydrogen evolution reaction catalyst comprising steps of (A) coating a carbon support in nickel chelates and molybdenum chelates, and (B) collapsing the nickel chelates and molybdenum chelates to yield a hydrogen evolution reaction catalyst.

Related aspects of the invention provide a method, e.g., as described above, wherein step (A) comprises dissolving nickel and molybdenum salts in water to form respective salt solutions, bringing the respective salt solutions to pH, adding a chelating agent to the respective salt solutions, and washing, filtering and drying chelate complexes resulting from addition of said chelating agent.

Further related aspects of the invention provide a method, e.g., as described above, comprising dissolving the chelate complexes in a solvent into which a carbon support is added, and forming a catalyst precursor by removing the solvent and drying.

Yet further related aspects of the invention provide a method, e.g., as described above, wherein step (B) comprises annealing the catalyst precursor.

Still yet further aspects of the invention provide a method, e.g., as described above, comprising forming the catalyst into an electrode.

Related aspects of the invention provide an electrode comprising a catalyst prepared according to the methods above.

Still other aspects of the invention provide an electrolyzer comprising a cathode formed of a catalyst prepared as discussed above.

Still further aspects of the invention comprise an anion exchange membrane electrolyzer with a cathode as described above.

The foregoing and other aspects of the invention are evident in the claims and in the description that follows and in the drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

A more complete understanding of the invention may be attained by reference to the drawings, in which:

FIG. 1 shows an x-ray diffraction pattern of a catalyst prepared according to the method of FIG. 6 ;

FIG. 2 shows a polarization curve of a catalyst prepared according to the method of FIG. 6 ;

FIG. 3 is a thermogravimetric analysis profile of cupferron in argon;

FIG. 4 is an X-ray diffraction pattern of a catalyst prepared according to the method of FIG. 7 ;

FIG. 5 shows how hydrogen evolution reactions are improved by comparing half-cell polarizations obtained before and after anodic cycling;

FIG. 6 shows a first method for fabricating an electrode according to the invention;

FIG. 7 shows a second method for fabricating an electrode according to the invention;

FIG. 8 shows an anion exchange membrane electrolyzer according to the invention; and

FIG. 9 shows a polarization curve of an anion-exchange membrane electrolysis cell using a cathode formed from a catalyst according to the invention.

DETAILED DESCRIPTION OF THE ILLUSTRATED EMBODIMENT

FIG. 6 shows a first method according to the invention for making a hydrogen evolution reaction (HER) catalyst, e.g., for use as a cathode in an electrolyzer, in which aminated metal salts are heated to produce Ni and Mo oxide particles on a carbon support which are subsequently reduced during alloying.

In step 10, nickel and molybdenum salts are dissolved in water, and ammonium hydroxide solution is added resulting in the corresponding aminated salts. Quantities and concentrations may be selected as discussed in connection with step 12, below.

In step 12, the solution of step 20 is added to a dispersion of a carbon support in a solvent that is miscible in water and has a high boiling point. In the illustrated embodiment, that carbon support comprises activated carbon in particle sizes of 0.5-3.5 microns and, preferably 1.58 microns, though, other support materials in other sizes may be used instead or in addition; the solvent can be any of chloroform, ethyl acetate, and/or hexanes, as selected in accord with high volatility and hydrophobicity. The amounts of solution (and the relative concentrations of the salts) and carbon support are selected to achieve a molar ratio of Ni:Mo between 9:1 and 8:2 and a resultant metal loading on the carbon support of 40-50%.

In step 14, the mixture of step 12 is heated to precipitate nickel and molybdenum oxides onto the carbon support. Selection of temperatures and precipitation times is within the ken of those skilled in the art in view of the teachings hereof.

In step 15, the solvent is removed, and the catalyst is dried. Selection of temperatures and drying times suitable to achieve those results, without degradation of the catalyst, is within the ken of those skilled in the art in view of the teachings hereof.

In step 16, the catalyst precursor produced in step 15 is annealed in hydrogen to reduce the oxide species. Alternative embodiments may utilize other techniques to effect such reduction.

In optional step 18, the catalyst can be formed into an electrode (in this case, a cathode) and, further optionally, assembled with an anion exchange membrane, anode and other componentry (e.g., gas diffusion layers, distribution plates and so forth) per convention in the art as adapted in accord with the teachings hereof, to form an electrolyzer of the type shown in FIG. 8 .

FIG. 7 shows a second method according to the invention for making a hydrogen evolution reaction (HER) catalyst, e.g., for use as a cathode in an electrolyzer, by coating a carbon support in metal-organic chelates (specifically, nickel chelates and molybdenum chelates) and collapsing the chelates to yield a catalyst.

In step 20, nickel and molybdenum chelates are formed using as chelating agents ethylenediamine tetraacetic acid (EDTA), cupferron, and/or cyclohexanediamine tetraacetic acid (CDTA). In the illustrated embodiment, each of the chelates are prepared using a 2:1 chelate to metal molar ratio, though, in other embodiments those ratios can vary from 1:1 to 4:1.

Formation of the chelates using those chelating agents can be accomplished in a manner known in the art; see, e.g., Vuyyuru, K. R., et al., Recovery of Nickel from Spent Industrial Catalysts Using Chelating Agents. Industrial & Engineering Chemistry Research, 2010. 49(5): p. 2014-2024, and Kumar, D., M. Singh, and A. Ramanan, Crystallization of Mo-EDTA complex based solids: Molecular insights. Journal of Molecular Structure, 2012. 1030: p. 89-94 (EDTA), Doan et al, supra, and Healy, W. and W. McCabe, Extraction of Submicrogram Amounts of Molybdenum with Cupferron-Chloroform Using Molybdenum-99. Analytical Chemistry, 1963. 35(13): p. 2117-2119 (cupferron), and Misra, M., et al., Alleviation of nickel-induced biochemical alterations by chelating agents. Fundamental and Applied Toxicology, 1988. 11(2): p. 285-292, and Kawakubo, S., R. Fukasawa, and M. Iwatsuki, Flow injection determination of ultratrace molybdenum in natural fresh and tap water samples by catalytic spectrophotometry. Journal of Flow Injection Analysis, 1997. 14: p. 25-38 (CDTA), the teachings of all of which are incorporated herein by reference. Thus, for example, nickel and molybdenum salts are dissolved separately in water and brought to alkaline (pH>7) and acidic (pH<7), respectively. An aqueous solution of the chelating agent is prepared at either an alkaline pH if it is intended for the nickel salt or neutral (pH=7) pH if it is intended for the molybdenum salt. The respective chelate solution is added dropwise (or otherwise) to the respective salt solution. When the addition of the respective chelating agent is completed, each of the respective resulting complexes is washed, filtered, and dried, e.g., under vacuum. This method differs from the previous method described by Doan et al, supra, by preparing a bimetallic catalyst as opposed to a monometallic one. In addition, the chelates are prepared separately and in the absence of the carbon support. Here, the dissolved chelates are loaded onto the carbon support via evaporation of the solvent.

In step 22, both of the metal-chelate complexes resulting from step 20 are dissolved in an organic solvent, added to a carbon support, and mixed thoroughly. In the illustrated embodiment, that carbon support may be selected and sized as discussed above in connection with step 12 of Method 1, and the metal-chelates are measured separately to yield and combined with the carbon support to achieve a molar ratio of Ni:Mo between 9:1 and 8:2 and a resultant metal loading on the carbon support of 40-50%. In these regards, the metal content of the chelates can be determined in a conventional manner known in the art, by methods such as inductively coupled plasma mass spectroscopy (ICP-MS) or energy dispersive spectroscopy (EDS), as adapted in accord with the teachings hereof.

In step 24, the solvent is drawn off the mixture formed in step 60 under vacuum and thoroughly dried at 800 C. under vacuum. In other embodiments solvent removal and drying can be accomplished at other temperatures and suitable times, as is within the ken of those skilled in the art in view of the teachings hereof, or alternatively by other techniques within the ken of those skilled in the art in view of the teachings hereof.

In step 26, the resulting catalyst precursor is milled to ensure an even mixture of the metal chelate and carbon support, as determined by EDS or ICP-MS and annealed in hydrogen to collapse the metal-chelated yielding the respective metal-oxide, reduce the metal oxide to M° oxidation state, and 3) alloy the metals.

Annealing in step 26 serves three purposes. Similar to annealing step 16 of method 1, heat treatment with 5% hydrogen (balance argon), for example, alloys the metals while reducing metal oxides. In addition, heat treatment collapses the metal chelate and, at suitably high enough temperatures (the selection of which is within the ken of those skilled in the art in view of the teachings hereof), degrades the chelating agent.

In optional step 28, the catalyst can be formed into an electrode (in this case, a cathode) and, further optionally, assembled with an anion exchange membrane, anode and other componentry (e.g., gas diffusion layers, distribution plates and so forth) per convention in the art as adapted in accord with the teachings hereof to form an electrolyzer of the type shown in FIG. 8 .

An advantage to the second method is the scalability and the speed at which the catalyst can be prepared. Large amounts of the metal chelates can be prepared ahead of time and stored until more catalyst is needed. This approach can also shorten the time to prepare catalyst.

The favorable metal/metal oxide obtained using this method yields a catalyst with a higher HER activity relative to the catalyst prepared using method 1 if subjected to cycling at anodic potentials. The process of cycling to anodic potentials transforms the metals into their corresponding oxides. An understanding of the purpose of these metal oxides and how they enhance the HER activity requires a brief review of the HER mechanism. HER occurs in alkaline media can occur via two different mechanisms described in Equations 1-3.

Volmer: H₂O+e−→H_(ads)+OH⁻  (1)

Heyrovsky: H₂O+e−+H_(ads)→H₂+OH⁻  (2)

Tafel: H_(ads)+H_(ads)→H₂  (3)

In the first method, a water molecule is adsorbed on a surface and the O-H bond is cleaved (Volmer). From here, the reaction proceeds either by the donation of a proton from a second water molecule (Heyrovsky) or recombine with a second adsorbed hydrogen atom (Tafel). An oxyphilic moiety would make the Volmer step described in equation 1 a more favorable. Therefore, the presence of surface oxides in this context would actually be desirable by attracting water molecules. However, if the concentration of metal oxides is too high, the passivated material would impede the electrochemical reactions. One of the novelties of this method is the fact by that cycling the catalyst to anodic conditions, a favorable amount of metal oxides are formed to attract water molecules while the regions of the catalyst surface covered by graphene are resistant to oxidation and remain catalytically active.

FIG. 5 shows how HER is improved by comparing half-cell polarizations obtained before and after anodic cycling. Results were obtained in 0.1 M KOH at room temperature using a metal loading of 50 μg/cm2. Reported voltages are versus RH

EXAMPLES

Example of Method 1: A NiMo/C HER catalyst was prepared in accord with the first method, i.e., that shown in FIG. 6 and described above in connection therewith. The solvent employed in step 12 was chloroform, and the weights and molar percentages of constituents were as shown in the table immediately below. Values were determined using EDS.

Weight % Molar % Carbon 55.8 Nickel 32.9 51.3 Molybdenum 11.26 10.7 Oxygen 38.0 Metal 44% n = 7, Weight % stddev 0.83%, Molar % stddev: 1.9%

In step 14, the mixture was brought to 140° C. for 90 minutes to remove the precipitate the Ni and Mo oxides onto the carbon support. And, in step 16, annealing was performed in the presence 5% hydrogen (balance argon).

FIGS. 1 and 2 show the x-ray diffraction pattern and polarization curves, respectively, of the NiMo/C HER catalyst prepared in the foregoing example. Results were obtained in 0.1 M KOH at room temperature using a metal loading of 50 μg/cm2. Reported voltages are versus RHE.

Example of Method 2: A NiMo/C HER catalyst was prepared in accord with the second method, i.e., that shown in FIG. 7 and described above in connection therewith. Here, cupferron was selected as the chelating agent in step 20, and the weights and molar percentages of constituents were as shown in the table immediately below. Values were determined using EDS.

Weight % Molar % Carbon 61.7 Nickel 33.4 52.7 Molybdenum 4.91 4.7 Oxygen 42.3 Metal 38% n = 7, Weight % stddev 0.55%, Molar % stddev: 3.1%

In step 26, annealing was performed in the presence of 5% hydrogen (balance argon).

FIG. 3 is a thermogravimetric analysis profile of the chelating agent cupferron in argon. The temperature increased from 30° C. to 600° C. at a rate of 10° C./min. The profile shows that cupferron loses 100% of its original mass at 250° C. The result of this degradation in the presence of metal is that the metal surface is partially covered by a thin layer of graphite.

FIG. 4 is an X-ray diffraction pattern of a catalyst prepared according to the foregoing example. In addition, a more pronounced graphite phase is observed in the catalyst's X-ray diffraction pattern relative to that prepared using method 1.

FIG. 9 shows a polarization curve of an anion-exchange membrane electrolysis cell (or electrolyzer) using a cathode formed from a catalyst according to the invention and, more particularly, formed from a catalyst prepared using method 2. The anode of the cell used a platinum-group metal catalyst. The cell operated at 90° C. and was fed 3% wt. potassium carbonate. The dashed line shows the voltage curve corrected for interfacial resistance determined by high-frequency resistance from impedance spectroscopy.

CONCLUSION

Described above are novel hydrogen evolution reaction catalysts, electrodes and electrolyzers based thereon and methods of fabrication according to the invention. It will be appreciated that the embodiments discussed above and shown in the drawings are examples of the invention and that other embodiments incorporating changes to those shown here also fall within the scope of the invention. 

In view of the foregoing, what is claimed is:
 1. A method of preparing a hydrogen evolution reaction catalyst comprising: A. co-precipitating nickel and molybdenum oxides on a carbon support to form a catalyst precursor, B. annealing the catalyst precursor in the presence of hydrogen to reduce the nickel and molybdenum oxides and, thereby, to yield a hydrogen evolution reaction catalyst.
 2. The method of claim 1, wherein step (A) comprises forming aminated salts of nickel and molybdenum.
 3. The method of claim 2, wherein step (A) comprises preparing a solution by adding the aminated salts to a dispersion of the carbon support in a solvent.
 4. The method of claim 3, comprising heating the solution to precipitate the nickel and molybdenum oxides onto the carbon support.
 5. An electrode comprising the catalyst prepared in claim
 1. 6. An electrolyzer comprising a cathode formed of the catalyst prepared in claim
 1. 7. The method of claim 1, comprising forming the catalyst into an electrode.
 8. A method of fabricating an electrode comprising A. co-precipitating nickel and molybdenum oxides on a carbon support to form a catalyst precursor, B. annealing the catalyst precursor in the presence of hydrogen to reduce the nickel and molybdenum oxides and, thereby, to yield a hydrogen evolution reaction catalyst, and C. forming the catalyst into an electrode.
 9. The method of claim 8, wherein step (A) comprises forming aminated salts of nickel and molybdenum.
 10. The method of claim 9, wherein step (A) comprises preparing a solution by adding the aminated salts to a dispersion of the carbon support in a solvent.
 11. The method of claim 10, comprising heating the solution to precipitate the nickel and molybdenum oxides onto the carbon support.
 12. A method of preparing a hydrogen evolution reaction catalyst comprising: A. coating a carbon support in nickel chelates and molybdenum chelates, and B. collapsing the nickel chelates and molybdenum chelates to yield a hydrogen evolution reaction catalyst.
 13. The method of claim 12, wherein step (A) comprises dissolving nickel and molybdenum salts in water to form respective salt solutions, bringing the respective salt solutions to pH, adding a chelating agent to the respective salt solutions, and washing, filtering and drying chelate complexes resulting from addition of said chelating agent.
 14. The method of claim 13, comprising dissolving the chelate complexes in a solvent into which a carbon support is added, and forming a catalyst precursor by removing the solvent and drying.
 15. The method of claim 14, wherein step (B) comprises annealing the catalyst precursor.
 16. An electrode comprising the catalyst prepared in claim
 12. 17. An electrolyzer comprising a cathode formed of the catalyst prepared in claim
 12. 18. The method of claim 12, comprising forming the catalyst into an electrode.
 19. A method of fabricating an electrode comprising A. coating a carbon support in nickel chelates and molybdenum chelates, and B. collapsing the nickel chelates and molybdenum chelates to yield a hydrogen evolution reaction catalyst, and C. forming the catalyst into an electrode.
 20. The method of claim 19, wherein step (A) comprises dissolving nickel and molybdenum salts in water to form respective salt solutions, bringing the respective salt solutions to pH, adding a chelating agent to the respective salt solutions, and washing, filtering and drying chelate complexes resulting from addition of said chelating agent.
 21. The method of claim 20, comprising dissolving the chelate complexes in a solvent into which a carbon support is added, and forming a catalyst precursor by removing the solvent and drying.
 22. The method of claim 21, wherein step (B) comprises annealing the catalyst precursor. 